This invention relates to the packaging and assembly of electrical components, and more particularly to the packaging and assembly for power converters with enhanced thermal management, high electrical interconnection density and low profile.
Please refer to FIG. 1. FIG. 1 shows one approach to package electric components in a power converter according to the prior art. As shown in FIG. 1, power-dissipating devices such as 9b, 9d and a magnetic element 9c are mounted directly on a metal base-plate 6 by a thermally conductive insulator 8 for better heat transfer. The power components are electrically connected with a PCB 5a by single-lateral leads 12. The magnetic element 9c is electrically connected to PCB 5a by leaded terminals. And, other necessary components such as 9a, 9e are mounted to one side or both sides of the PCB 5a. The assembly may be encapsulated within a capsule (not shown), which acts as a heat spreader and mechanical support.
One drawback of the packaging art in FIG. 1, however, is that since the power-dissipating devices 9b, 9d are electrically connected with the PCB 5a by the single-lateral leads 12, it will be difficult to keep these devices co-planar during the mounting process, which would make the overall manufacturing process more complex, and thus would undesirably increase the manufacturing cost. Meanwhile, the leaded electrical connections of power dissipation devices as well as magnetic element will cause high interconnection parasites, which will result in additional connection resistance loss and poor converter performance especially for high current converter packaging. Moreover, the power-dissipating devices 9b, 9d are comparatively in a bigger footprint, which would inevitably cause a lower power density packaging.
U.S. Pat. No. 5,973,923 discloses another prior art approach to package electronic components in a power converter as shown in FIG. 2. In the packaging, a thermal connection from a power-dissipating device 22 to the outside is constructed, in which copper coated vias 42 are prepared by the manufacturing process of the PCB 28. The metal slug 43 of the power-dissipating device 22 is mounted onto the top pad of the copper coated vias 42, and a metal base-plate 32 is attached to the bottom pad of the copper coated vias 42 through thermally conductive insulators 30, 104. Through these copper coated vias 42, the heat is transferred from the power-dissipating device 22 to the other side of the PCB 28, and further transferred to the metal base-plate 32. The magnetic element 26b is also mounted on the PCB 28, and is connected with metal base-plate 32 by a thermally conductive soft pad 34 to enhance the heat transfer.
One drawback of the packaging art in FIG. 2, however, is that since the power-dissipating device 22 is mounted onto top-side of the copper coated vias 42, and the bottom-side of the copper coated via is occupied by the thermal connections 30, 104, the assembly reduces the electrical interconnection density of the board 28. Moreover, heat from the power device 22 will meanwhile be transferred by the copper coated vias 42 to the board 28, and thereby increases temperature rise of the board 28.
With increasing power density and current output capability of the power converter packaging, much more power-dissipating devices are to be mounted onto PCB in parallel to satisfy high current output and meanwhile to well control the conduction power losses. Thus more and more PCB lands are to be occupied by power-dissipating devices. Moreover, the power losses from power-dissipation devices and its conductive trace increase abruptly with the increase of the output current. With the increase of power density and output current, how to manage the heat from these power-dissipating devices and how to shorten the interconnection trace therebetween become very critical for power converter packaging.
It is therefore an object of the present invention to propose a packaging technology for power converters to achieve enhanced thermal management, high interconnection density, improved electrical performance as well as low cost.
To address the above-discussed deficiencies of the prior art, the present invention provides, a package for a power converter, a method of packaging a power converter and a sub-package arrangement employed in the package of the present invention. The package includes: (1) a multi-layer circuit board having a first electrically conductive surface and a second electrically conductive surface opposite to the first electrically conductive surface; (2) a heat spreader having at least a heat transfer surface; and (3) a sub-package having at least a power-dissipating chip, which is disposed between one of the electrically conductive surfaces of the multi-layer circuit board and the heat transfer surface of the heat spreader, having a bare top heat-slug in contact with the heat transfer surface of the heat spreader through a thermally conductive insulator to provide a thermally conductive pathway therebetween, and having a plurality of symmetric leads in contact with one of the electrically conductive surfaces of the multi-layer circuit board to provide an electrically conductive pathway therebetween, wherein the plurality of symmetric leads and the heat-slug are disposed in opposite orientations so that the electrically conductive pathway is separate from the thermally conductive pathway.
In one embodiment of the present invention, the plurality of symmetric leads can also be replaced by balls to meet some electrical connection requirements.
In the present invention, the sub-package, magnetic elements and other passive components of the package are fully mounted onto a multi-layer circuit board.
The unique aspect of this packaging concept is that the power-dissipating chips of the package, which are typically power semiconductors, are assembled in advance into the sub-package, therefore enabling the package to have high interconnection density. The sub-package has a bare top heat-slug and a plurality of symmetric leads (or balls), wherein the plurality of symmetric leads and the heat-slug are disposed in opposite orientations. In one embodiment of the present invention, the heat-slug is up-facing and the leads are down-facing. Of course, the heat-slug can also be down-facing and the leads be up-facing.
In the present invention, the sub-package is disposed between the electrically conductive surface of the multi-layer circuit board and the heat transfer surface of the heat spreader. In this way, the first electrically conductive surface of the multi-layer circuit board is adapted to be electrically connected with the leads of the sub-package to provide an electrically conductive pathway therebetween. The heat transfer surface of the heat spreader is adapted to be thermally connected with the heat-slug of the sub-package through a thermally conductive insulator to provide a thermally conductive pathway therebetween. The heat from the magnetic elements can also be transferred to the ambient by the attached heat spreader. Therefore, the electrically conductive pathway is separate from the thermally conductive pathway, which achieves enhanced thermal management and improved electrical performance.
In one embodiment of the present invention, the sub-package further includes control chips and passive chips. For the reason of increased packaging density and improved electrical performance, the power-dissipating chips, control chips and passive chips can be combined into one sub-package together. More specifically, the sub-package with only one chip can be simplified to be a standard surface-mounting package with a bare top up-facing heat-slug.
Another aspect of the present invention provides a method of packaging a power converter including: (1) providing a multi-layer circuit board having at least an electrically conductive surface; (2) providing a heat spreader having at least a heat transfer surface; and (3) disposing the sub-package having at least a power-dissipating chip between the electrically conductive surface of the multi-layer circuit board and the heat transfer surface of the heat spreader, the sub-package having bare top up-facing heat-slug and a plurality of symmetric down-facing leads, whereby the electrically conductive surface of the multi-layer circuit board is adapted to be electrically connected with the leads of the sub-package to provide an electrically conductive pathway therebetween; the heat transfer surface of the heat spreader is adapted to be thermally connected with the heat-slug of the sub-package with at least a power-dissipating chip through a thermally conductive insulator to provide a thermally conductive pathway therebetween; and the electrically conductive pathway is separate from the thermally conductive pathway.
Another aspect of the present invention provides a sub-package. In order to satisfy miscellaneous applications, a derivative art is proposed for the sub-package. In the art, the leaded discrete power-dissipating devices have a bare bottom heat-slug and are electrically and thermally connected with the heat-slug of the sub-package. The control devices and passive components are electrical connected with the power-dissipating devices by the conductive trace on the bottom surface of the heat-slug of the sub-package. The sub-package is electrically connected to the mother-board of a power converter by a plurality of symmetric leaded terminals or balls, and is thermally connected to the outside by the heat-slug of the sub-package. Therefore, the electrical conductive pathway of the sub-package is separate from the thermal conductive pathway.
The present invention may best be understood through the following description with reference to the accompanying drawings, in which: